Sludge from sewage and wastewater treatment plants represents a serious disposal problem. Currently, sewage sludge is incinerated, disposed of in or used as alternative daily cover in landfills and used as fertilizer. Incineration, however, requires large amounts of fuel because sewage sludge contains a high volume of water, giving it a highly negative heat value. Moreover, use as daily cover in a landfill or fertilizer has disadvantages because sludge generates undesirable odors and can lead to contamination of soil or landfill from, for example, heavy metals or pathogens.
Furthermore, there is a growing wave of public support for renewable energy, popularly called “Green Power.” Several well-known companies, according to Power magazine for May 2003, including General Motors, IBM, Dow Chemical and Johnson & Johnson, have announced plans to purchase a portion of their power requirement from “green” sources.
In addition, several state legislatures are requiring power marketers to phase in specific and increasing percentages of power from renewable sources. New York has mandated that state agencies must buy 25% of their power from renewable sources by 2013. Currently 19.3% of the energy produced in New York is generated from renewable sources (New York Public Service Commission). California has passed legislation requiring that 20% of utilities' electricity in the state be produced from renewable sources by 2017. Similar governmental efforts are taking place in Europe and elsewhere.
As the foregoing demonstrates, the disposal of sewage has become increasingly expensive and difficult while at the same time there is a growing need for renewable energy and green power. A need therefore exists for processes and apparatuses to cleanly and economically dispose of sewage.